Thermo Fisher Scientific CyberScan 6000 Series Meters v.2 - Manual (Page 90)

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Appendix: pH Theory
PH6000, PC6000, PH6500, PC6500 and PCD6500 meters
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pH Electrodes
The electrode system consists of two half cells: a pH indicating electrode, which is primarily
responsive to the acidity (the hydronium ion concentration-tion) of a solution, and a reference
electrode, which provides a constant voltage and completes the electrical circuit.
Traditional pH indicating electrodes use a tip made of pH sensitive glass. Inside this electrode
is a buffer solution with a fixed pH and ionic strength. A silver wire coated with silver chloride is
immersed inside this internal solution, and establishes electrical contact between the solution and
the meter. The voltage associated with this wire and the voltage associated with the pH of the
internal solution and the inside wall of the pH sensitive glass tip remain constant. Therefore,
changes in voltage from this electrode result from the voltage developed between the solution
and the outside of the glass tip.
If the pH sensitive glass tip or membrane is to function properly, it must be hydrated. A dry or
dehydrated electrode membrane will not respond properly to changes in acidity. pH electrodes
are also effected by changes in temperature, and the presence of other ions. Temperature effects
can be countered by temperature compensation functions on the pH meter. While temperature
compensation will not allow you to predict what the pH of the sample is at another temperature, it
will permit you to accurately assess the pH at whatever temperature you are presently working.
pH sensitive membranes are also sensitive, though to a lesser extent, to other ions than the
hydronium ion. For example, most pH glasses are somewhat sensitive to sodium ion as well. For
some pH glasses, this means an error as high as 0.5 pH units in highly alkaline, high-sodium
solutions.
Reference electrodes typically consist of three elements: an internal reference electrode, a filling
solution, and a reference junction through which the filling solution can "flow", and provide
electrical contact with the sample and the internal reference electrode. The most convenient
internal reference electrodes are made from a metal (such as silver or mercury) and its sparingly
soluble salt (silver chloride or mercurous chloride). The filling solution is most often a
concentrated solution of potassium chloride. Most problems with reference electrodes are
associated with the interruption or blockage of flow of the potassium chloride fill solution through
the reference junction.
The classic electrode pair, separate pH indicating and reference half cells, offer you unmatched
versatility to match the needs of your sample. This approach makes for the highest accuracy, as
well as low replacement costs, since usually only one of the pair is broken or malfunctioning.
However, the pH indicating half-cell and the reference half-cell can be merged into one
electrode: a combination electrode. Combination electrodes are quite popular for they offer
distinct advantages in convenience and compactness. Some combination electrodes also
incorporate an ATC probe into their body also, providing temperature readout and compensation
with meters equipped with these features. Newer combination electrodes are available in which
the glass pH membrane sensor has been replaced with a field effect transistor or FET.